1. Field of the Invention
The present invention relates to an optical transceiver module applicable to a diplex or a triplex type optical transceiver for Ethernet PON or GPON and the like and, more specifically, to a planar lightwave circuit type optical transceiver module using a wavelength division multiplexing (WDM) scheme.
2. Discussion of Related Art
Recently, a planar lightwave type optical transceiver module has been developed up to a commercialized level, and it is expected that a low-cost planar lightwave type optical transceiver module will be widely used in optical communication subscriber networks.
The optical transceiver module can be used as a diplex transceiver for a broadband passive optical network (BPON) communications using a couple of upstream and downstream wavelengths or, as a triplex transceiver using an upstream wavelength and two distinguished downstream wavelengths. The BPON is expected to be developed into gigabit Ethernet PON or GPON in the very near future.
A conventional planar lightwave circuit type optical diplex transceiver module includes a light-emitting unit, a light-receiving unit, and single mode waveguides, and they are integrated into a substrate made of ceramic material or silicon material such as a Si wafer. An optical waveguide is formed on a substrate to face the light-emitting unit, and the other optical waveguide is formed on the substrate to face the light-receiving unit. Here, the optical waveguides are arranged adjacent to each other at a predetermined portion, and a thin film filter (TFF) is arranged at the predetermined portion to transfer optical signals to the light-receiving unit or from the light-emitting unit depending on the wavelength.
In the optical transceiver module arranged as described above, the thin film filter is inserted into a trench formed on the substrate. However, the trench is typically formed using a sawing process, so that it is difficult to be finely formed due to a characteristic of the process. In other words, a location and a gap of the trench may be formed irregularly.
A reflection plane of the thin film filter should be exactly aligned to a place at which two optical waveguides are adjacent to transmit the optical signals transmitted from an optical fiber to the light-receiving unit with a minimal loss. When the trench is not exactly formed as described above, however, the reflection plane of the thin film filter can be out of a proper position leading to a severe loss of the optical signal.
To maintain the optical signal loss depending on the location of the thin film filter below 1 dB, there is difficulty in that the location of the thin film filter should be adjusted within an error range of ±1 μm for an excessive case (when an incident angle is large). Moreover, for a case where more than two thin film filters should be inserted, as in the triplex type optical transceiver, the loss depending on the location of the filter is more critical.